1. Field of Invention
The present invention relates to movable micromirrors and micromirror arrays for direct-view and projection displays. U.S. Pat. Nos. 5,835,256 and 6,046,840 to Huibers, and U.S. patent application Ser. No. 09/617,419 to Huibers et al., the subject matter of each being incorporated herein by reference, disclose micro-electromechanical devices (MEMS) for steering light beams, such as in an optical switch, and/or for forming a display (e.g. a projection display). A common feature is a mirror element which is movable so as to deflect light through different angles, depending upon the mirror element""s tilt angle. In one type of conventional direct view or projection display system, an array of reflective mirror elements are provided for producing an image. Typically the mirror elements are square.
2. Related Art
One type of micromirror array is disclosed in Kim et al. xe2x80x9cThin-film Micromirror Array (TMA) for High Luminance and Cost-competitive Information Display Systemsxe2x80x9d, SPIE Photonics West, EI99 where an array of mirrors are microfabricated on a MOS addressing circuitry substrate. Each (square) mirror is addressed by the active matrix transistor array. Each mirror is held by a center post above a top electrode and a bottom electrode, which are in turn disposed above a supporting layer and the active matrix.
In Hornbeck""s xe2x80x9cDigital Light Processing for High-Brightness, High-Resolution Applicationsxe2x80x9d, a similar arrangement is disclosed. A (square) mirror is held on a post above a yoke and hinge and a mirror address electrode, which are in turn disposed above a yoke address electrode and a CMOS substrate.
In U.S. Pat. No. 6,038,056, the mirror elements are again square and the center post, also square, is disposed so that corresponding sides of each post and the mirror are parallel. The light source is placed at a 45 degree angle to the mirror array. One problem with such an arrangement, is that the light source must be placed at an angle from the mirror array (in the X, Y and Z axes) which increases the cost of the illumination optics and the overall size of the projection system.
In order to minimize light diffraction along the direction of switching and more particularly light diffraction into the acceptance cone of the projection optics, in the present invention, mirrors are provided which are not rectangular (xe2x80x9crectangularxe2x80x9d as used herein including square mirrors). Diffraction as referred to herein, denotes the scattering of light off of a periodic structure, where the light is not necessarily monochromatic or phase coherent Also, in order to minimize the cost of the illumination optics and the size of the display unit of the present invention, the light source is placed orthogonal to the rows (or columns) of the array, and/or the light source is placed orthogonal to a side of the frame defining the active area of the array. The incident light beam, though orthogonal to the rows (or columns) and/or side of the active area, should not, however, be orthogonal to sides of the individual mirrors in the array. Orthogonal sides cause incident light to diffract along the direction of mirror switching, and result in light xe2x80x98leakagexe2x80x99 into the on-state even if the mirror is in the off-state. This light diffraction decreases the contrast ratio of the mirror.
The present invention optimizes the contrast ratio of the mirror array so that when mirrors are in their off-state they send minimal light to the spatial region where light is directed when mirrors are in their on-state. More specifically, the present invention comprises a particularly located light source and incident light beam and particularly designed mirrors in the array, which minimizes light diffracted into the acceptance cone of the projection (or viewing) optics, so as to provide an improved contrast ratio. The arrangement/design of the present invention also minimizes non-reflective areas in the array, by allowing for a tight fit of mirrors and a large fill factor with low diffraction from the off to the on-state, even when the array is illuminated along the axes of mirror periodicity. Namely, the design optimizes contrast ratio through angular sides non-parallel to the mirror""s axis of rotation and optimizes fill factor through hinges that require a relatively small amount of area and allow neighboring mirrors to tile together with little wasted non-reflective area.